1. Field of the Invention
This invention relates to a mask for use in the lithography process of semiconductor devices, and more particularly to a mask for use in a stitching exposure technique for forming a mask pattern by overlapping at least two masks especially in the case that a chip size is larger than an exposure area exposed by exposure equipment.
2. Description of the Prior Art
The stitching exposure is a technique for separately exposing at least two masks to fabricate a semiconductor chip. In this technique, an overlapping region is normally placed between the masks so that the patterns of the separately exposed region are connected together.
A conventional mask used in a stitching exposure will be explained with reference to FIGS. 1 and 2.
FIGS. 1 and 2 show an upper mask 100 and a lower mask 200 each of which has a light blocking pattern 20 such as a chrome pattern attached on a transparent wafer substrate such as a quartz wafer. In use, a photosensitive layer deposited over the wafer is first exposed using the upper mask 100, and secondly exposed using the lower mask 200, and then developed by a photolithography process to form a photosensitive pattern.
At this stage, respective overlapping regions 30 are placed between the lower portion of the upper mask 100 and the upper portion of the lower mask 200 in order to align the masks 100 and 200.
However, in separately exposing the masks 100 and 200 as described above, negative effects may occur in the region where the photosensitive pattern should maintain since the intensity of light (i.e., a beam energy) passing through the overlapping region becomes higher than that of other regions.
FIG. 3 is a plane view illustrating a photosensitive pattern 50 which is obtained by exposing the photosensitive layer deposited on the upper surface of the wafer using the masks 100 and 200, and then by developing by the photolithography process. As shown in FIG. 3, the line width of the photosensitive pattern 50 is narrowed in the overlapping region of the masks. As the photosensitive pattern is narrowed as described above, the photosensitive pattern may collapse or be cut off in the worst condition during the developing and cleaning process.